By virtue of having a center of assymetry at the C-2 carbon, the enantiometric difference in metabolism of oxyprenolol, a beta-adrenergic blocking agent widely used in Europe, has been investigated. The metabolism of oxyprenolol by liver microsomes requires oxygen, TPNH and is inhibited by carbon monoxide atmosphere, piperonyl butoxide and SKF 525-A. Thus it appears to be catalyzed by cytochrome P-450. The comparative studies of the corresponding metabolism by two isomers, the S and R forms of oxyprenolol, by liver microsomes from rat, mouse, guinea pig, hamster and rabbit demonstrate clearly that the stereospecificity of the dealkylation and ring hydroxylation depends on animal species.